Closed pond system for wet process phosphate plants

ABSTRACT

Closed pond system or process for eliminating the conventional pond systems of wet process phosphoric acid complexes, and to remove fluorine process gas streams, wherein liquid effluents from wet process phosphoric acid complexes, including scrubber liquor from gas scrubbing operations of the complex, are cooled and clarified, and the liquid from the clarifier recycled to the phosphate complex, the sludge or slurry being filtered to remove solids, the solids being calcined to drive off fluorinecontaining gases. The calcined solids consist principally of sodium fluoride (NaF) which is recycled to the clarifier overflow to be mixed with the liquid returned to the phosphate complex. In the phosphate complex the liquid from the clarifier is used to scrub plant gases, and is mixed with other scrubber liquors from the plant, and recycled again to the cooling tower and clarifier. The HF in the effluent plant liquids is converted to SiF4 by maintaining an excess of SiO2, and the NaF reacts with the SiF4 to form Na2SiF6. The Na2SiF6 is calcined to produce NaF and gaseous SiF4, the latter being scrubbed with water to form H2SiF6 of commercial quality.

United States Patent 1 Hartig Jan. 7, 1975 CLOSED POND SYSTEM FOR WETPROCESS PHOSPHATE PLANTS Rufus G. Hartig, 230 Hillsboro Hotel Building,Dover, Fla. 53602 22 Filed: July 24,1972

21 Appl. No.: 274,222

Related U.S. Application Data [63] Continuation-impart of Ser. No.60,226, Aug. 3,

1970, Pat. No. 3,720,757.

[76] Inventor:

OTHER PUBLICATIONS Industrial and Engineering Chemistry Vol. 53, No. 9,Sept. 1961, 705-707.

Primary Examiner-Oscar R. Vertiz Assistant Examiner-Gregory A. HellerAttorney, Agent, or Firm-Carl B. Fox, Jr.

[57] ABSTRACT Closed pond system or process for eliminating theconventional pond systems of wet process phosphoric acid complexes, andto remove fluorine process gas streams, wherein liquid effluents fromwet process phosphoric acid complexes, including scrubber liquor fromgas scrubbing operations of the complex, are cooled and clarified, andthe liquid from the clarifier recycled to the phosphate complex, thesludge or slurry being filtered to remove solids, the solids beingcalcined to drive off fluorine-containing gases. The calcined solidsconsist principally of sodium fluoride (NaF) which is recycled to theclarifier overflow to be mixed with the liquid returned to the phosphatecomplex. In the phosphate complex the liquid from the clarifier is usedto scrub plant gases, and is mixed with other scrubber liquors from theplant, and recycled again to the cooling tower and clarifier. The HF inthe effluent plant liquids is converted to SiF by maintaining an excessof SiO and the NaF reacts with the SiF to form Na SiF The Na SiF iscalcined to produce NaF and gaseous SiF the latter being scrubbed withwater to form H SiF of commercial quality.

2 Claims, 2 Drawing Figures PATENTED T1975 3. 859.423

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NN- ml- CLOSED POND SYSTEM FOR WET PROCESS PHOSPHATE PLANTS Thisapplication is a continuation-in-part of applicants application Ser. No.60,226, filed Aug. 3, 1970, now U.S. Pat. No. 3,720,757 entitled ClosedPond System for Wet Process Phosphate Plants.

BACKGROUND OF THE INVENTION This process is designed to eliminate allcontamination by gaseous fluorine liberated in phosphate rock processingfrom the waters used to slurry gypsum, in wet process phosphoric acidcomplexes, and to remove fluorine from process gas streams. In addition,various other benefits are obtained including fluorine recovery as asaleable product, elimination of liming costs, higher P recovery, andthe like.

In current practice, by-product gypsum from wet process phosphoric acidcomplexes is slurried with water and pumped to a pond of vast acreage inwhich the gypsum settles out and the supernatant water is cooled inponds and recycled to process for use as scrubbing water to removefluorine from process gas streams, as condensing streams in barometriccondensers, and to slurry gypsum. The fluorine content of this waterbuilds up to appreciable concentrations presenting undesirable problems,such as surface water and ground water contamination by fluorine.Addition of lime to these waters is frequently resorted to in order tocontrol pollution of surface waters into which the plant waters maybecome intermingled.

By the novel process herein presented, it is possible to eliminate thistype of fluorine contamination by completely eliminating the use ofgypsum slurry water in scrubbing and processing. All fluorinecontaminated scrubbing liquor is handled and stored in impervious, linedequipment separate from the gypsum slurry system.

The liquid solution circulated through this process and the complex hasa pH of 4.5-4.7, and therefore has no fluorine vapor pressure becausethe fluorine therein is present as the two stable salts, Na SiF and NaF.This condition precludes the presence of any fluorine in the air exitsof cooling towers, and in other equipment. The total gaseous fluorineemission from the entire closed pond system will be less than pounds perday, for a phosphate complex of the size wherein the volume of liquideffluent is in the 16,000 gallons per minute range.

Other objects and advantages of the process herein disclosed will appearfrom the following detailed description of a preferred embodiment,reference being made to the accompanying drawings.

BRIEF DESCRIPTIONS OF THE DRAWINGS FIGS. 1A and 1B are flow diagramswhich together show a preferred embodiment of the process.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, theeffluent leaving the phosphate complex enters the process at flowline10, which leads into catch basin 11. The liquid contains approximately0.66 percent fluorine, and is at a temperature of between 100F and 120F.The fluorine is present as Na SiF which has been formed by the reactionbetween NaF and SiF A slight excess of reactive Si0 is maintained in therecirculating liquid stream to convert any HF to SiF, (4HF SiO SiF 2HO). The liquid also contains a small amount of NaF in solution.

The liquid is pumped from catch basin 11 by cooling tower lift pump 12,the flow being through flowline 13 containing valve 14 and, upon leavingthe pump, through flowline 17 which delivers the liquid to the top ofcooling tower 18.

The liquid passes downwardly through cooling tower 18, while air isdrawn in through plural side inlets 19 to pass upwardly through misteliminator 20, the air being drawn upwardly by a blower 21 driven by amotor, not shown. The cooling tower may be packed or equipped withbaffles, or the like, in any suitable manner. Any suitable coolingtowermay be substituted.

The cooled liquor which leaves the cooling tower by gravity throughflowline 23 is at a temperature of F, maximum. The liquid is deliveredthrough flowline 23 into the inlet 24 of clarifier 25. The clarifier maybe of any suitable form. Clarifier 25 as shown includes paddles or drags26 which rotate in order to collect the thickened slurry at the lowercenter of the clarifier. The clarifier has a downwardly convergingbottom or floor, and has therearound an overflow trough 27 in flowcommunication with an overflow sump 28. Liquid overflow from theclarifier exits through flowline 29 containing valve 30, pumped by pump31 through flowline 32 which delivers the overflow liquid from theclarifier back to the phosphate complex, wherein the liquor is used forscrubbing and/or other plant purposes, the liquid being circulatedthrough the complex to be returned to the herein described processthrough flowline 10.

Fresh water is added as necessary to the overflow sump 28 throughflowline 37.

The overflow liquor from the clarifier contains approximately 0.6%fluorine, present as Na SiF and a small excess of NaF in solution. Thisconcentration of fluorine is approximately 0.06% less than theconcentration of fluorine in the liquid entering the process throughflowline 10. The excess fluorine in flowline 10 is derived from thephosphate complex scrubbing and washing operations.

The slurry leaving clarifier 25 at the center bottom thereof exitsthrough flowline 39 having valve 40, being moved by pump 41 deliveringthe slurry through flowline 42 having valve 43 to settler 45. Settler 45has a downwardly converging conical bottom, and is equipped with a motordriven paddle or drag 46. The overflow liquid from settler 45 isreturned through flowline 47 to be mixed with the cooling tower liquidoutput in flowline 23.

Fresh water is added to the slurry from clarifier 25 delivered throughflowline 42 to settler 45 through a flowline 48.

The concentrated slurry discharged from the center bottom of settler 45is delivered through flowline 49 having valve 50, compelled by pump 51through flowline 52 including valve 53 to a filter 54. The filter 54 isindicated in the drawing as being rotary drum filter, but other suitablefiltration apparatus may be suitably used. The filtrate from filter 54is discharged through flowline 55 by gravity to be mixed with the liquidoverflow from clarifier 25 at overflow sump 28.

A water slurry of Si0 may be introduced to the clarifier overflow sump28 through a flowline 53 in order to maintain the SiO concentration inthe liquid recirculated to the phosphate complex.

The solid materials collected at filter 54 are delivered by conveyor 58to the feed 59 ofa calciner 60. The calciner 60 is indicated in thedrawing as being of the rotary drum, countercurrent flow type, but othersuitable apparatus for this purpose may be substituted. Fuel and air areburned at burner 62 of calciner 60, the flue gas passing toward the leftthrough the calciner to be discharged through flowline 63. v In calciner60, Na SiF is decomposed at a temperature of 1,350 F to produce solidNaF and gaseous SiF The gaseous SiF, is discharged from calciner 60through flowline 63 with the flue gases from the burner. A portion ofthe liquid in flowline 32 is diverted through flowline 65 to beintroduced into NaF slurry tank 66, the amount introduced beingcontrolled by valve 67. Slurry from tank 66 is circulated throughflowline 69 including valve 70 by pump 71 which delivers the slurrythrough flowline 72 including valve 73 through the discharge 76 ofcalciner 60. Calcined NaF is discharged from the calciner to be mixedwith the circulating slurry at 76. The slurry-solids mixture is passedthrough flowline 77 to the slurry tank 66.

A portion of the slurry in flowline 72 is passed through flowline 79 tobe delivered into the clarifier overflow sump 28. A portion of theslurry passing through flowline 79 is circulated through flowline 80having valve 81 through the bottom of calciner feed 59, to be deliveredthrough flowline 82 for mixture with the slurry mixture in flowline 77.In this way, material at the calciner feed dust hood may be mixed withthe slurry in slurry tank 66.

The flow through flowline 79 delivers make-up NaF to the closed pondcirculating liquid returned to the phosphate complex through flowline32.

The gases discharged from calciner 60 pass through flowline 63, throughscrubber 87, and therefrom through flowline 88 to a second scrubber 89.After passing through scrubber 89, the gases are passed through flowline91 to a third scrubber 92, and thence through flowline 93 to a fourthscrubber 95. From scrubber 95 the gases pass through flowline 96 toblower 97, and thence through flowline 98 havivng valve 99 to stack 100.

Fresh water is introduced to scrubber 92 to wash the gases, throughflowline 103. The scrubber liquor from tower 92 exits through valve 104and flowline 105 through pump 106 which delivers part of the liquid inrecirculation through line 107 having valve 108, and which delivers apart of the liquid through flowline 109, flow through which iscontrolled by valve 110, into scrubber 89 wherein the scrubber liquor iscontacted with the gases passing through the scrubber. The scrubberliquor is discharged from scrubber 89 through valve 114 and flowline115, and is recirculated to the top of scrubber 89 through flowline 116,being propelled therethrough by pump 117. Flow through flowline 116 iscontrolled by valve 118. Scrubber slurry is withdrawn from this systemthrough flowline 120 having valve 121.

Scrubber liquor from scrubber 87 is delivered through flowline tosettler 126. Settler 126 has a downwardly converging conical bottom andhas rotating paddles or drags 127. Slurry from settler 126 is dischargedfrom the bottom center of the settler through valve 129 and flowline 130to be moved by pump 131 through flowline 132 having valve 133. Liquidoverflow from the settler is discharged through valve 135 and flowline136, pump 137 moving the liquid through flowline 138 having valve 139 tobe recirculated through the scrubber 87. Slurry from clarifier 25 inflowline 42 can be diverted through valve 140 and flowline 141 to thefeed of settler 126. A portion of liquor from flowline 65 is passedthrough valve 143 and flowline 144 to scrubber 87.

Flowline 145 branching from flowline 65 carries overflow liquid fromclarifier 25 to the top of scrubber 95, flowline 145 having a controlvalve 146. Recirculating liquor in tower 95 drains through valve 147 andflowline 148 to be pumped by pump 149 through recirculation flowline150, flowline 150 having valve 151. Scrubber liquor is withdrawn fromflowline 150 through valve 153 and flowline 154 to be delivered into thefeed inlet of clarifier 25. Liquid condensing in blower 97 is drawnedthrough flowline 156 back to scrubber 95. The scrubber has a misteliminator 157 through which the exit gas passes into flowline 96.

The gases leaving calciner 60 through flowline 63 contain SiF, derivedfrom decomposition by heat of Na SiF according to the reaction, Na siF2NaF SiF In the described system, scrubber 87 is a recycling scrubberprovided primarily to remove fluoride dusts prior to recovery of thefluorine, and to convert the NaI-I PO to I-I PO by the reaction Theslurry from settler 126 delivered through flowline 132 is passed througha heat exchanger 161 (cooler). Liquid from flowline 65 is passed throughflowline 162 to cool the slurry and is returned through flowline 163which communicates with flowline 154. The cooled slurry is passedthrough flowline 165 into agitated retention tank 166, from which it isdelivered through valve 167 by pump 168 and through flowline 169 havingvalve 170 into filter 172. Filter 172 is indicated as being a rotarydrum filter, but other types of filtration equipment may be substituted.The filtrate from filter 172 is delivered through flowline 174 tobarometric seal tank 175, from which it passes through valve 176 and ispumped by pump 177 through flowline 178 which delivers recoveredphosphoric acid, as explained below, to the phosphate complex. Thephosphoric acid solution may be delivered through valve 180 and flowline181 to settler 126. The cake from filter 172 is sluiced with liquor ofany suitable nature and source or with water at 183 to be deliveredthrough flowline 184 to mix with the scrubber liquor returned toclarifier 25 through flowline 154.

In the liquid circulation stream through cooling tower l8, clarifier 25,and the phosphate complex, phosphoric acid present in the liquor reactswith NaF and SiO, to form NaI-I PO which has a pH of approximately 4.7which is in the pH range of the circulating liquor. Thechemical'reaction is:

The NaI-I PO concentration of the circulating liquor can be allowed tobuild up to 5 to 20 percent P 0 P 0 in the form of NaH PO, iscontinuously removed from the circulating liquor which is delivered toscrubber 87. Gaseous SiF from calciner 60 reacts to form phosphoric acidas shown in the following chemical reactions:

4NaI-I PO 3SiF 2H O 2Na SiF 411 1 0 SiO The circulating liquor iscontrolled by checking the pH and if the pH is at 4.7 or above there isa sufficient excess of NaF and SiO present. If the pH falls below 4.7there is a deficiency of SiO or NaF in the system. Insoluble Na SiF andSiO which enter the phosphoric acid recovery area through flowline 132,are filtered out by filter 172 and recycled to the clarifier 25. Thephosphoric acid obtained according to the above reaction is deliveredthrough flowline 178 to the phosphate complex, or used in other suitablemanner.

Since the scrubber liquor of scrubber 87 is saturated Table 1 Typicaloperating conditions Flow rate Flow rate through through Temperaflowline flowline ture NaHzPO, Na Sifi, NaZSiFm S.Cu.Ft.l H PO, NaF,SiFi Flowline Gal/Min. F (as P (as F) NaF SiOz Lbs/Min. H2O Min. SiF,(as P 0 Lbs./Min. Lbs/Min. HQSiFE 6 13,000 106 10.0 0.5 SLXS SLXS 1713,000 106 10.0 0.5 SLXS SLXS 23 13,000 90 10.0 0.5 SLXS SLXS 79 82 15.052 180 184 58 184 63 600 15.9 6600 4.8 102 132 160 40 10 93 7500 Trace100 Nil 138 300 0.7 10 178 23 10 196 33 150 20 with respect to SiF SiF,(gas) continuously passes through flowline 88 to scrubber 89, where theSiF is recovered as a water solution of H SiF Any remaining SiF isrecovered in scrubber 92, and tail scrubber 95 picks up any residue sothat the gases passing out of stack 100 are essentially fluorine free.The scrubber liquor passing through flowline 120 is delivered through afilter 190, which is indicated to be of the rotary drum type, but forwhich any suitable type of filtration apparatus may be substituted. Thefiltrate passes through barometric leg flowline 191 to seal tank 192.This filtrate is a water solution of H SIF which is delivered throughvalve 193, pump 194, and valve 195 through flowline 196 to storage. Thissolution is a valuable commercial product.

The filter cake from filter 190, consisting of SiO, in pure form isdelivered by conveyor 197 to slurry tank 198 to which water is addedthrough flowline 199 controlled by valve 200. The SiO slurry isdelivered through valve 203 by pump 204 through valve 205 and flowline206 to the plant, for addition of SiO to the plant process where neededor for other use. A part of the SiO slurry is delivered through valve208 and flowline 53, previously described, to the clarifier overflowsump 28.

Approximately 16,000 gallons per minute of plant effluent is deliveredin a typical phosphate complex through flowlines l0 and 17 to coolingtower 18, the

liquid being of a specific gravity of approximately 1.1

By conversion of the fluorine components (SiF and HF) in the phosphatecomplex effluent to Na SiF by maintaining an excess of NaF and SiO inthe liquid circulating through the process from the phosphate complex,and separating the Na SiF from the circulating stream, the fluorinevalues are effectively removed from the phosphate complex effluent. Bycalcining the Na SiF fluorine is recovered by water scrubbing as H SiFand NaF is recovered for re-use. Phosphate values are recovered as H POor may be recycled to the plant as NH H PO The gases from the processare essentially free of fluorine, and no water solutions are disposed ofto result in contaminations of surface and ground waters.

While a preferred embodiment of the invention has been shown in thedrawings and described, many modifications may be made by a personskilled in the art without departing from the spirit of the invention,and it is intended to protect by Letters Patent all forms of theinvention falling within the scope of the following sh m I claim:

1. Process for minimizing the fluorine vapor pressure of scrubber andwash liquors of phosphate complexes, fluorine in the liquors beingprincipally in the form, SiF comprising maintaining an excess of NaF insaid liquors in the scrubbing towers and washing location of. thephosphate complex so that substantially all SiF, which is absorbed byand dissolved in said liquors in the phosphate complex is immediatelyconverted to solid Na SiF in suspension according to the chemical reac--tion, SiF, 2 NaF Na SiF flowing said liquors from said scrubbing towersand wash locations, separating Na SiF from said liquors, calcining theseparated Na SiF to form NaF and SiF, according to the 2. The processaccording to claim 1, wherein any HF in said liquors in the phosphatecomplex is converted to H SiF by addition of Si to said liquors,according to the chemical reaction, 6HF SiO H SiF vapor pressure of saidliquors is maintained low, and 2H O.

returning said liquors to the scrubbing towers and wash locations ofsaid phosphate complex for reuse.

1. PROCESS FOR MINIMIZING THE FLUORINE VAPOR PRESSURE OF SCRUBBER ANDWASH LIQUORS OF PHOSPHATE COMPLEXES, FLUORINE IN THE LIQUORS BEINGPRINCIPALLY IN THE FORM, SIF4, COMPRISING MAINTAINING AN EXCESS OF NAFIN SAID LIQUORS IN THE SCRUBBING TOWERS AND WASHING LOCATION OF THEPHOSPHATE COMPLEX SO THAT SUBSTANTIALLY ALL SIF4 WHICH IS ABSORBED BYAND DISSOLVED IN SAID LIQUORS IN THE PHOSPHATE COMPLEX IS IMMEDIATELYCONVERTED TO SOLID NA2SIF6 IN SUSPENSION ACCORDING TO THE CHEMICALREACTION, SIF4+2 NAF-> NA2SIF6, FLOWING SAID LIQUORS FROM SAID SCRUBBINGTOWERS AND WASH LOCATIONS, SEPARATING NA2SIF6 FROM SAID LIQUORS,CALCINING THE SEPARATED NA2SIF6 TO FORM NAF AND SIF4 ACCORDING TO THECHEMICAL REACTION, NA2SIF6 (HEAT) 2NAF + SIF4, RETURNING AT LEAST APORTION OF THE NAF TO SAID LIQUORS TO MAINTAIN SAID EXCESS THEREIN ANDABSORBING THE SIF4 IN WATER TO FORM H2SIF6 IN SOLUTION, WHEREBY THEFLUORINE VAPOR PRESSURE OF SAID LIQUORS IS MAINTAINED LOW, AND RETURNINGSAID LIQUORS TO THE SCRUBBING TOWERS AND WASH LOCATIONS OF SAIDPHOSPHATE COMPLEX FOR REUSE.
 2. The process according to claim 1,wherein any HF in said liquors in the phosphate complex is converted toH2SiF6 by addition of SiO2 to said liquors, according to the chemicalreaction, 6HF + SiO2 -> H2SiF6 + 2H2O.